The use of doped oxide films as sources of impurities for solid-state diffusion in the fabrication of semiconductor devices is well established in the electronics art. Many attempts have been made in the past to provide uniform dopant ladened films which may be used to transfer N-type and P-type impurities into semiconductor wafer material.
One approach has been to form a doped glassy layer on a semiconductor wafer by using silicone polymers which are admixed with dopant sources. The mixture of silicone polymer and dopant is coated onto a wafer and heated to produce a glassy dopant laden film. Upon further heating the dopant is diffused, in the solid state, from the glassy film into the semiconductor wafer. Such silicone based coating methods are examplified in U.S. Pat. Nos. 3,084,079; 3,798,081 and 3,834,939. Such methods which rely on silicone polymer matrices suffer from several shortcomings. The silicon-carbon bond in the silicone polymer is not subject to hydrolytic degradation. Rather, such a bond must be thermally degraded in the presence of oxygen, at high temperatures. Circumstances exist wherein high temperature degradation of the polymer matrix containing the dopant are not advisable. Further, silicone polymers can cause significant carbonaceous residue deposition upon thermal degradation which is not precisely controlled. Such carbonaceous residue buildup adversely affects the uniformity of semiconductor doping upon use of such silicone polymers.
Another approach to semiconductor doping is represented by U.S. Pat. Nos. 3,615,943; 3,837,873; 3,915,766; 3,928,225; and 4,152,286. Each of these patents represents a method of semiconductor doping wherein a silicon tetra-alkoxide is converted via acetic anhydride (or a lactic acid residue) to a mixed silicon acetate alkoxide. Such conversion eliminates the problem of premature volatilization of silicon material prior to conversion to the glass since silicon alkoxides are volatile while silicon acetates are less volatile. The mixed silicate alkoxide is combined with a dopant source. Subsequently, the mixed alkoxide and dopant source are heated to degrade the mixed alkoxide silicate into a silica based glass with dopant contained therein. Such methods also suffer from the adversities well known when organic compenments (such as acetate and lactate residues) are thermally degraded as described above. Methods employing silicon acetate alkoxide material do exhibit the advantage of reduced volatility of the silicate upon initial heating to form a glassy matrix.
Other attempts to provide dopant sources for semiconductor wafer uses are disclosed in U.S. Pat. Nos. 3,789,023 and 4,243,427. U.S. Pat. No. 3,789,023 discloses a liquid diffusion dopant source for semiconductor doping involving dissolving dopants in an aqueous-alcoholic solution by refluxing the starting materials. Subsequently, to the refluxed material are added alcohols, an ethyl acetate and tetraethylorthosilicate.
U.S. Pat. No. 4,243,427 discloses a mixed solution approach to dopant source provision. The patent discloses a coating composition formed by heating mono-aluminum phosphate; adding methyl alcohol to the hot solution; cooling the solution and subsequently mixing tetraethylorthosilicate therewith. The solutions must be used rapidly once mixed due to the inherent instability of the resultant two component solution.
The present invention provides a process and composition for semiconductor wafer doping which provides an extremely uniform dopant film or coating which is derived from very stable solutions of starting material. Further the present invention provides a liquid dopant source which is well suited to spin-coating applications and is maintained in the stable solution phase during all steps of the process, up to actual heating of the semiconductor wafer. No carbonaceous residues or nonuniformity of doping problems are experienced.